Field plotter

ABSTRACT

A field plotter for conductive sheet analog simulation having constant current sources and calibration circuitry that permits the resistance of the model to be readily accommodated and enables the operator to normalize the contour voltages by establishing the relative contour value of the point on the sheet of highest potential. The sheet is supported on an easel and connections thereto are made from the front with pins that pierce both the sheet and the easel, leads then being attached to the pins at the rear of the easel. A hold-down frame automatically makes contact with the perimeter of the sheet for those models utilizing a grounded outer boundary.

United States Patent Dalke FIELD PLOTTER Inventor: George W. Dalke,Lawrence, Kans.

Assignee: Interpretation Systems Incorporation, Lawrence, Kans.

Filed: Oct. 1, 1970 Appi. No.: 77,065

References Cited UNITED STATES PATENTS Wolf ..235/6l.6 B Clark ..324/71Bruce ..235/6l.6 B

[451 Sept. 26, 1972 Primary Examiner-Michael J. Lynch Attorney-Schmidt,Johnson, Hovey & Williams [5 7] ABSTRACT A field plotter for conductivesheet analog simulation having constant current sources and calibrationcircuitry that permits the resistance of the model to be readilyaccommodated and enables the operator to normalize the contour voltagesby establishing the relative contour value of the point on the sheet ofhighest potential. The sheet is supported on an easel and connectionsthereto are made from the front with pins that pierce both the sheet andthe easel, leads then being attached to the pins at the rear of theease]. A hold-down frame automatically makes contact with the perimeterof the sheet for those models utilizing a grounded outer boundary.

10 Claims, 7 Drawing Figures PATENTED SEP 26 1972 SHEET 1 0F 2 W I I 1 w36 19 7' TORNE VS.

PATENTEU 3.694. 743

sum: 0F 2 INVENTOR, George W. Dalke.

,QTTORNEYS- FIELD PLOTTER This invention relates to improvements infield plotters for conductive sheet analog simulation.

The field plotter is an electrical device which facilitates theinvestigation of a wide variety of physical phenomena. Most physicalsituations involving energy distribution follow the general mathematicalformulas defined by Laplaces and Poissons equations. Such situations canbe translated into electrical parameters on the field plotter andinvestigated in an analog fashion. Applications include diversetwo-dimensional problems such as those relating to heat transfer,electrostatic fields, population density, and other phenomena where thedistribution of energy is involved.

Field plotting techniques have heretofore been used with analog models,but several problems have been presented that has made general use offield plotters rather impractical, particularly for the scientist orinvestigator who is not oriented in the electrical arts. A primaryproblem is that of interpreting the results once equipotential lineshavebeen plotted. The values of voltages along these lines are rathermeaningless unless their interrelationship is known and understood,otherwise the solution to the problem or the presentation of theanalysis escapes interpretation. Furthermore, the complexity of theplotting apparatus must be held to a minimum from both the standpointsof cost and ease of operation, and a problem has also been presentedwith respect to the making of solid electrical connections to the mediumof the model. Additionally, a field plotter for general use should becapable of accommodating a very wide range of model resistances so thatfew limitations will be placed on the investigator insofar as the natureof the problem which the plotter is capable of handling.

It is, therefore, an important object of the present invention toprovide a field plotter for analog simulation which overcomes theproblems and deficiencies discussed hereinabove, resulting in aninstrument suitable for diverse applications and which may be easilyoperated by the untrained investigator.

Another important object of the invention is to provide a field plotteras aforesaid in which the equipotential lines of the analog arenormalized with respect to a known reference, thereby facilitating arapid interpretation of the results obtained.

Still another important object of this invention is to provide a fieldplotter as aforesaid which is capable of accommodating models of widelyvarying resistances so as to place no significant limitation on theinvestigative applications of the instrument.

A further and important object of this invention is to provide a fieldplotter as aforesaid in which positive electrical connections to themodel medium are made in a simple and straightforward manner in order tofacilitate rapid setup of the model, and wherein such connections in noway interfere with the plotting of the equipotential lines of theanalog.

In the drawings:

FIG. 1 is a frontal, perspective view of the field plotter showing asimple model with equipotential lines plotted thereon;

FIG. 2 is an enlarged, fragmentary, cross-sectional view of the easelshowing two of the connector pins in contact with the paper sheet;

FIG. 3 is a detail view illustrating the finger stock ground strip;

FIG. 4 is a detail view of the contact head of a con- I DETAILEDDESCRIPTION Referring initially to FIGS. 15, the field plotter has acase 10 provided with a hinged backplate 12 that, when extended, forms astand for maintaining an easel portion 14 of the case 10in an inclinedposition. A control panel 16 is located on the left side of the easel 14as viewed in FIG. 1, and includes a calibrator knob 18, a contourselector knob 20, a null meter 22, nine input adjustment knobs 24, anon-off switch 26, and an indicator light 28 associated therewith.

The ease] 14 includes a backing member in the form of a corkboard 30having a front face which receives a paper sheet 32. The sheet 32comprises the resistive medium of the model under investigation andanalysis, a carbon coated cellulose paper being preferred. A rectangularhold-down frame 34 is hinged at 36 on the case] 14. A fingerstock strip36 is mounted on the backside of the frame 34 and extends completelyaround the frame to present a rectangular grounding strip which is incontact with the sheet 32 at its perimeter when the frame 34 is in theposition illustrated, held in place by a pair of latches 38. Thecorkboard 30 has a peripheral recess that is filled by a cushion strip40 of rubber or the like, the strip 40 directly opposing the groundingstrip 36.

Electrical contact is made to the sheet 32 by a plurality (as many asnine) of connectors 42, each of which comprises a pin 44 having acontact head 46 recessed at 48 to present an annular contact surface 50surrounding the longitudinal axis of the pin 44. The pins 44 pierce thesheet 32 and the cork board 30 and thus protrude from the rear thereof,connections being made to the pins 44 by releasable clips 52 from whichleads 54 extend to the circuitry of the apparatus described hereinafter.

Referring to FIG. 6, a step down transformer 56 has its primaryconnected to power terminals 58 via the onoff switch 26. A full wavebridge rectifier 60 is connected to the secondary of the transformer 56,and a filter capacitor 62'is' connected across the rectifier output. Thepositive side of the power supply output is at chassis ground asindicated by the symbols. The negative output is delivered along a lead64 to the emitter resistors 66 of nine constant current sources 68 ofidentical configuration.

A diode 70 has its cathode connected to the negative supply lead 64 andits anode connected via a lead 71 to the lower end of a potentiometer 72in each of the sources 68. The active component of each source 68 is anNPN transistor 74 having its base connected to a tap 76 on therespective potentiometer 72. Each of the input adjustment knobs 24 onthe control panel 16 is connected to a corresponding tab 76 to providefor manual adjustment thereof.

A calibrate potentiometer 78 is connected by resistors 80 and 82 betweenthe positive ground and a lead 84 which is an extension of the negativesupply lead 64. A contour potentiometer 86 is also connected betweenground and lead 84, two series resistors 88 and 90 being interposedbetween potentiometer 86 and lead 84. The tap 92 on the calibratepotentiometer 78 is connected to the common junction of resistors 88 and90 and is also connected to the base of an NPN emitter followertransistor 94. A lead 96 connects the emitter of transistor-94 to eachof the current source potentiometers 72. A resistor 97 is connectedbetween the collector of transistor 94 and ground.

A current limiting resistor 98 is connected to the tap 100 of thecontour potentiometer 86, and is in series with a resistor 102 whichsets the sensitivity of the null meter 22. A pair of parallel,oppositely poled diodes 104 protect the meter movement from excessivecurrent. A jack 106 is connected to he side of the meter 22 opposite" tothe series resistors 98 and 102, the jack 106 receiving a plug 108(FIG. 1) which, via an accompanying lead 110, is connected with avoltage sensing probe 112. A. graphite lead 114 serves as the probe tipso that marks may be made on the paper sheet 32 at points of appropriatevoltage. The probe 112 is a drafting pencil which is modified byconnecting the lead 110 to the end of the metallic pencil body oppositethe lead 114.

Nine leads 116 extend from corresponding current sources 68 to nineterminals designated 1 through 9 respectively. One or more of the leads54 (FIG. 2) extend from these terminals to the pin or pins 44 utilizedto form a particular model. The nine knobs 24 carry numericaldesignations of 1 through 9 and, likewise, the heads 46 of the pins 44are numbered 1 through 9 as is clear in FIG. 5. Accordingly, aparticular knob 24is identified with a particular pin 44 of the model.The calibration knob 18 adjusts the tap 92 of the calibrationpotentiometer 78; and the contour selector knob adjusts the tap 100 ofthe contour potentiometer 86.

OPERATION Regardless of the nature of the model, the basic operation ofthe field plotter is the same. In FIG. 1 a simple model is illustratedhaving three energy sources represented by the contact heads 46 of threeof the pin connectors 42. The situation under investigation might, forexample, be to determine the interaction of the electric fieldsgenerated by three parallel, current-carrying conductors, represented bythe three contact heads 46. It should be understood that the fingerstockstrip 36 carried by the hold-down frame 34 is connected to chassisground, thus current flow in the paper sheet 32 is from each of thecontact heads 46 outwardly to the circumscribing strip 36. For a papersheet 32 having a length of l6 inches, the resistance of the carboncoated face thereof between two lines of contact extending along theopposed end edges may, for example, be on the order of 4,000 ohms.However, the resistance of the model between each of the contact heads46 and the ground boundary may vary widely depending upon the nature ofthe model, as will subsequently be appreciated when FIG. 7 is discussed.The object is to plot the equipotential lines 118 such as illustrated inFIG. 1, utilizing the lead tip 1 14 of the pencil probe 112. Theseequipotential or contour lines constitute an analog simulation andrepresent the solution to the model.

The pin connectors 42 numbered 1, 2, and 3 would be used to form thesimple model illustrated in FIG. 1, together with the grounding strip 36which is automatically brought into electrical contact with the sheet 32when the hold-down frame 34 is locked in place with the latches 38. Thefinger contacts of the strip 36 depress the sheet 32 by virtue of theprovision of the cushion strip 40 at the edge of the corkboard backingmember 30. Thus, solid electrical contact is assured by the strip 36 atthe perimeter of the sheet 32. Provision may be made for providinggrounded boundaries or points other than the boundary defined by thestrip 36, depending upon the requirements of a given model.

In forming the model illustrated in FIG. 1, the pin connectors 42identified by the numerals 1, 2, and 3 on their contact heads 46 areselected and inserted through the paper sheet 32 and the corkboard 30 atthe desired points. It will be assumed for purposes of illustration thatthe two adjacent, uppermost contact heads 46 in FIG. 1 are labelled 1and 2 and that each represents an energy level equal to 5 on a relativescale of from 0 to 10, and that the lowermost contact head 46 is number3 and has an energy level equal to 8. The leads 54 from the number 1, 2,and 3 output terminals of the constant current sources 68 are connectedto the protruding pins 44 of corresponding designation, thus theoperator knows that the input knobs 24 numbered 1, 2, and 3 control theenergy inputs to the correspondingly numbered contact heads 46.Accordingly, knobs 24 numbered 1 and 2 are set at 5 and the third knobis set at 8. (The relative scale of 0 to 10 corresponds to 0 to 100percent of the current available at the output of each of the constantcurrent sources 68, the 100 percent condition occurring when the tap 76of the potentiometer 72 is set such that the full resistance of thepotentiometer 72 is connected between the base of the transistor 74 andthe lead 71.)

At this juncture, it is instructive to note that each of the constantcurrent sources is of identical circuit configuration. Representativevalues are 10,000 ohms for the potentiometer 72 and 1,000 ohms for theresistor 66, with a DC output level from the power supply of 38 voltswith respect to chassis ground. The voltage drop across the diode 70 is0.6 volts, and the voltage drop between the base and emitter of thetransistor 74 yields a voltage at the emitter of 0.6 volts below thebase voltage. The potentiometer 72, therefore, is connected between avoltage on lead 71 of 0.6 volts below the power supply output and avoltage on lead 96 determined by the setting of the tap 92 on thecalibration potentiometer 78. A very high gain transistor is selectedfor the transistor 74, thus its collector current can be assumed to beequal to its emitter current. Accordingly, the output current from anindividual source 68 is equal to the percentage of the resistance of thepotentiometer 72 between tap 76 and lead 71, multiplied by biaa Vealohms where V is the voltage on lead 71 and V is the calibration voltageappearing on lead 96. Thus, the output current is a direct function ofthe potentiometer setting and will always be a certain percentage of thetotal current available as selected by the setting of the tap 76, butthe absolute value of the output current will be determined by thecalibration voltage on lead 96. This is true for all of the currentsources 68, therefore the calibration voltage on lead 96 determines theab solute values of the currents appearing at the outputs of the sources68, but their relative values, expressed as a percentage of the maximum,remain unaffected.

Returning to the exemplary model illustrated in FIG. 1, once the knobs24 controlling the current delivered to the three contact heads 46 areset at their relative values as discussed hereinabove, a calibrationprocedure is followed which establishes the relative contour value ofthe point of highest potential in the modeled system. This value isnormally selected as the 100 percent contour so that, for convenience,lesser contour levels will be expressed directly as a percentage of thehighest potential in the system, thereby eliminating the subsequent needto normalize results in terms of percentages of a reference value.

To proceed with calibration, the contour selector knob 20 is set at 100percent, which corresponds to locating the tap 100 of the contourpotentiometer 86 at the lower end of the potentiometer resistor.Accordingly, maximum negative voltage with respect to ground appears atthe tap 100 and is sensed by the meter 22. The operator is aware thatthe highest potential will be at one of the contact heads 46, and inmore sophisticated models the heads are individually probed until thehighest potential is found. In probing from head to head, the tip 114 ofthe probe 112 is placed on a particular contact head 46 and, if themeter 22 deflects in a direction indicating that the voltage at suchhead is greater than the 100 percent contour volt- I age at tap 100, thecalibration knob 18 is adjusted to move the tap 92 in the calibrationpotentiometer 78 away from ground to decrease the output levels of thecurrent sources 68 until a balance is obtained (meter movement centeredon 0). This is repeated as needed as the other heads are probed until,ultimately, the meter 22 will indicate a balance of voltages only whenthe one head is probed having the highest potential in the system. Allother headswill then indicate a voltage less than the 100 percentcontour voltage.

When calibration has been completed, the operator is ready to plot thecontour lines that represent the solution to the model. For practicalpurposes, contour lines or less will generally suffice. The particularcontour levels selected for plotting should be chosen to provide themost information possible with regard to the specific model. Sincetheselevels will vary with the model, some experimentation may benecessary.

In plotting a contour, the contour level selected is first set on thecontour selector knob 20. The three contour lines 118 illustrated inFIG. 1 are 20, 40, and 43 percent contours respectively from the outsideto the inside. With the knob 20 at a particular setting, the tip 114 ofthe probe 112 is placed in contact with the sheet 32 and is movedthereover until the meter 22 indicates a balance. This point is markedand then sub sequent points where the meter balances are marked,whereupon a line connecting these points represents the contour levelindicated by the knob setting.

Referring to FIG. 7, a more complex model is illustrated along with theplotted analog solution. The problem is geographical and is concernedwith the influence of barriers and expediters on population potential.Four cities are centered at 119, 120, 122 and 124. These are thelocations of the energy inputs (connector pins), the relative energylevels thereof being 10.0, 2.5, 2.5, and 10.0. Accordingly, thenorthwest and southeast cities centered at 119 and 124 have four timesthe population of the two towns centered at 120 and 122. The two townsare located by a river 126, and a road 128 bridges the river and joinsthe city 119 and the town 120. The river 126 is formed on the model bysevering the paper sheet to interrupt electrical continuity, thus nocurrent can flow from one bank of the river to the other. The road 128is formed by a paint applied to the sheet having greater conductivitythan the carbon coated surface. The 10, 20, 30, 40, 50, and 60 percentcontour lines of the solution are designated, and the broken linesrepresent the 45 percent contour levehThe solution reveals that thegeneral population potential north of the river is greater, owing to theroad 128. The area south of the river reflects less interaction amongthe cities because of the absence of roads.

I By virtue of the calibration procedure discussed hereinabove, thecontour lines on the model of FIG. 7 are read directly in percentages,thus the solution may be readily interpreted once the contour lines areplotted, without the need for mathematical calculations. Furthermore,the resistance of the model has changed from that of the paper sheetalone due to the river 126 and the road 128. However, this constitutesno obstacle to effective use of the present invention since it isreadily adapted to a wide range of model resistances.

In this latter regard, it should be noted that the tap 92 on thecalibrate potentiometer 78 is connected to the junction of the resistors88 and 90, thus the contour voltage at the tap of the contourpotentiometer 86 is affected by the setting of the calibration tap 92.During the calibration procedure, if the resistance of the model is toolow for the calibration setting, as the tap 92 is moved toward groundthe maximum current available from'the current sources 68 increases, andthe contour voltage at the tap 100 simultaneously decreases. Withincreased current input to the model, the IR drops are greater. Thecontour voltage at tap 100, however, has decreased so that the currentinput needed to match the 100 percent contour voltage is not as great aswould otherwise be the case. This same dual compensating action isachieved when the resistance of the model is high, since the contourvoltage increases as the calibration tap 92 is moved away from ground todecrease the current input to the model. The emitter follower 94 servesto transfer the voltage on the calibration tap 92 to the current sources68 without loading the calibration circuit.

As the model to be analyzed becomes more complex and more current inputsare used, it becomes increasingly clear that the single calibrationcontrol provided by the knob 18 and associated movable tap 92 provides ameans of normalizing a solution that would otherwise require extensivemathematical interpretation. Although the possibility of employing ninecurrent inputs to the model is illustrated herein, any number can beprovided by simply adding additional constant current sources.Regardless of the settings of the taps 76 on the various potentiometers72 of the current sources 68, the ratios of their current outputs remainconstant regardless of the variation of the absolute values thereofrequired to properly calibrate the apparatus prior to plotting thecontours. It should be understood that this ability to simultaneouslyvary the output current levels of the sources 68, of itself, renders theapparatus capable of handling a wide range of model resistances evenwithout the dual compensation arrangement discussed above. However,simultaneous variation of the contour voltage as the calibration tap 92is adjusted is preferred due to the enhanced compensation.

Other important aspects of the present invention relate to the manner inwhich electrical connections are made with the paper sheet 32. Sinceconstant current sources are utilized, the contact resistance of theheads 46 against the sheet 32 is not critical. Accordingly, the pinconnectors 42 may simply be pushed through the sheet 32 and corkboard 30until the annular contact surfaces 50 engage the coated surface of thepaper. By virtue of their annular configuration, the ease of makingsolid contact with the coating is enhanced and relatively low resistanceconnections are assured. The pin 44 of each connector is frictionallyheld by the corkboard 30, thereby retaining the contact surface 50 inengagement with the sheet 32.

Furthermore, the leads to the pin connectors do not in any way interferewith the plotting of the equipotential contour lines. Since allconnections are made at the rear of the easel 14, the front of the ease]is entirely unobstructed so that points may be rapidly plotted and theconnecting contour lines drawn.

It should be understood that the constant current sources 68 illustratedherein are purely exemplary and may be replaced with sources of greaterstability for applications where high accuracy is desired. Operationalcharacteristics of the sources 68 discussed hereinabove assume that thetransistors 74 are neither saturated or near cutoff. The apparatus willnot calibrate when one of the transistors 74 is saturated, and thecutoff region may be avoided by setting the potentiometers 72 at 10percent or above. This permits the operator to utilize current sourcesettings of from 10 to 100 percent without regard to absolute values ofcurrent, thus the ratios of the energy inputs to the model may be ashigh as 10 to l and still be within the capabilities of the sources 68.

Having thus described the invention, what is claimed as new and desiredtov be secured by Letters Patent is:

1. Apparatus for plotting an electrical analog of a physical situation,said apparatus comprising:

a resistive medium adapted for modeling in accordance with thecharacteristics of said situation, said medium comprising a sheetmember;

a pierceable backing member having a front face receiving said sheetmember thereon and supporting the latter;

a plurality of input signal sources, each of which is provided withmeans for selectively setting its output at a predetermined levelrelative to the other sources;

a plurality of connectors for establishing electrical contact withselected contact points on said medium, each of said connectorscomprising an electrically conductive pin adapted to pierce said membersand provided with a contact head in engagement with said sheet memberwhen the pin has pierced both of said members and extends therethrough;

means for coupling said connectors with the outputs of correspondinginput signal sources for flow of currents along the medium from theconnectors in contact therewith, whereby voltages are developed in themedium in accordance with the magnitudes and interaction of saidcurrents, said coupling means being connectable at the rear of saidbacking member with the pins extending therethrough;

contour plotting means for providing a selectively variable contourvoltage, and having a voltage sensing probe for contacting said mediumand means for indicating the amplitude relationship between the sensedvoltage and a selected contour voltage; and

calibration means coupled with said input signal sources for varying theselected output levels thereof while maintaining the ratios of saidselected levels constant, whereby the selected levels may be adjusted tonormalize the selected contour voltages by establishing the relativecontour value of the point on the medium of highest potential.

2. The apparatus as claimed in claim 1,

said calibration means having acontrol for simul taneously effectingsaid variation of the selected output levels while maintaining theratios thereof constant.

3. Apparatus for plotting an electrical analog of a physical situation,said apparatus comprising:

a resistive medium adapted for modeling in accordance with thecharacteristics of said situation, said medium comprising a sheetmember;

a pierceable backing member having a front face receiving said sheetmember thereon and supporting the latter;

a plurality of constant current sources, each of which is provided withmeans for selectively setting its output current at a predeterminedlevel relative to the other current sources;

a plurality of connectors for establishing electrical contact withselected contact points on said medium, each of said connectorscomprising an electrically conductive pin adapted to pierce said membersand provided with a contact head in engagement with said sheet memberwhen the pin has pierced both of said members and extends therethrough;

means for coupling said connectors with the outputs of correspondingcurrent sources for flow of currents along the medium from theconnectors in contact therewith, whereby voltages are developed in themedium in accordance with the magnitudes and interaction of saidcurrents, said coupling means being connectable at the rear of saidbacking member with the pins extending therethrough;

contour plotting means for providing a selectively variable contourvoltage, and having a voltage sensing probe for contacting said mediumand means for indicating the amplitude relationship between the sensedvoltage and a selected contour voltage; and

calibration means coupled with said current sources for .varying theselected output current levels thereof while maintaining the ratios ofsaid selected levels constant, whereby the selected levels may beadjusted to accommodate the resistance of the model and to normalize theselected contour voltages by establishing the relative contour value ofthe point on the medium of highest potential.

4. The apparatus as claimed in claim 3,

said calibration means having a control for simultaneously effectingsai'd variation of the selected output current leads while maintainingthe ratios thereof constant.

5. The apparatus as claimed in claim 3,

said calibration means being operable to provide a calibration signaland having selectively operable control means for varying the magnitudeof said calibration signal,

said current sources having a common control input for receiving saidcalibration signal, and each of said sources delivering its outputcurrent in accordance with the magnitude of said calibration signal.

6. The apparatus as claimed in claim 5,

said contour plotting means including a control for varying said contourvoltage and selectively setting the latter at either a predeterminedmaximum amplitude or a lesser amplitude which is a known percentage ofthe maximum amplitude.

7. The apparatus as claimed in claim 6,

said indicating means being operable to compare the amplitudes of theselected contour voltage and said sensed voltage and indicate when saidamplitudes are equal.

8. The apparatus as claimed in claim 6,

said calibration means being coupled with said contour plotting meansfor increasing or decreasing the predetermined maximum amplitude of saidcontour voltage as said selected output current levels are decreased orincreased respectively.

9. The apparatus as claimed in claim 3, the contact head of each of saidpins having an annular contact surface in surrounding relationship tothe piercing axis of the pin.

10. Apparatus for plotting an electrical analog of a physical situation,said apparatus comprising:

a resistive medium adapted for modeling in accordance with thecharacteristics of said situation, said medium comprising a sheetmember, there being a backing member receiving said sheet member thereonand supporting the latter;

a hold-down frame shiftable to a position clamping the perimeter of thesheet member between the frame and the backing member;

a plurality of input signal sources, each of which is provided withmeans for selectively setting its output at a predetermined levelrelative to the other sources; a common return circuit coupled with theoutputs of said input signal sources and including a contact elementcarried by said frame and engaging the perimeter of the sheet member incircumscribing relationship thereto when the frame is in said position;

a plurality of connectors for establishing electrical contact withselected contact points on said medimeans for coupling said connectorswith the outputs of corresponding input signal sources for flow ofcurrents along the medium from the connectors in contact therewith,whereby voltages are developed in the medium in accordance with themagnitudes and interaction of said currents;

contour plotting means for providing a selectively variable contourvoltage, and having a voltage sensing probe for contacting said mediumand means for indicating the amplitude relationship between the sensedvoltage and a selected contour voltage; and

calibration means coupled with said input signal sources for varying theselected output levels thereof while maintaining the ratios of saidselected levels constant, whereby the selected levels may be adjusted tonormalize the selected contour voltages by establishing the relativecontour value of the point on the medium of highest potential.

1. Apparatus for plotting an electrical analog of a physical situation,said apparatus comprising: a resistive medium adapted for modeling inaccordance with the characteristics of said situation, said mediumcomprising a sheet member; a pierceable backing member having a frontface receiving said sheet member thereon and supporting the latter; aplurality of input signal sources, each of which is provided with meansfor selectively setting its output at a predetermined level relative tothe other sources; a plurality of connectors for establishing electricalcontact with selected contact points on said medium, each of saidconnectors comprising an electrically conductive pin adapted to piercesaid members and provided with a contact head in engagement with saidsheet member when the pin has pierced both of said members and extendstherethrough; means for coupling said connectors with the outputs ofcorresponding input signal sources for flow of currents along the mediumfrom the connectors in contact therewith, whereby voltages are developedin the medium in accordance with the magnitudes and interaction of saidcurrents, said coupling means being connectable at the rear of saidbacking member with the pins extending therethrough; contour plottingmeans for providing a selectively variable contour voltage, and having avoltage sensing probe for contacting said medium and means forindicating the amplitude relationship between the sensed voltage and aselected contour voltage; and calibration means coupled with said inputsignal sources for varying the selected output levels thereof whilemaintaining the ratios of said selected levels constant, whereby theselected levels may be adjusted to normalize the selected contourvoltages by establishing the relative contour value of the point on themedium of highest potential.
 2. The apparatus as claimed in claim 1,said calibration means having a control for simultaneously effectingsaid variation of the selected output levels while maintaining theratios thereof constant.
 3. Apparatus for plotting an electrical analogof a physical situation, said apparatus comprising: a resistive mediumadapted for modeling in accordance with the characteristics of saidsituation, said medium comprising a sheet member; a pierceable backingmember having a front face receiving said sheet member thereon andsupporting the latter; a plurality of constant current sources, each ofwhich is provided with means for selectively setting its output currentat a predetermined level relative to the other current sources; aplurality of connectors for establishing electrical contact withselected contact points on said medium, each of said connectorscomprising an electrically conductive pin adapted to pierce said membersand provided with a contact head in engagement with said sheet memberwhen the pin has pierced both of said members and extends therethrough;means for coupling said connectors with the outputs of correspondingcurrent sources for flow of currents along the medium from theconnectors in contact therewith, whereby voltages are developed in themedium in accordance with the magnitudes and interaction of saidcurrents, said coupling means being connectable at the rear of saidbacking member with the pins extending therethrough; contour plottingmeans for providing a selectively variable contour voltage, and having avoltage sensing probe for contacting said medium and means forindicating the amplitude relationship between the sensed voltage and aselected contour voltage; and calibration means coupled with saidcurrent sources for varying the selected output current levels thereofwhile maintaining the ratios of said selected levels constant, wherebythe selected levels may be adjusted to accommodate the resistance of themodel and to normalize the selected contour voltages by establishing therelative contour value of the point on the medium of highest potential.4. The apparatus as claimed in claim 3, said calibration means having acontrol for simultaneously effecting said variation of the selectedoutput current leads while maintaining the ratios thereof constant. 5.The apparatus as claimed in claim 3, said calibration means beingoperable to provide a calibration signal and having selectively operablecontrol means for varying the magnitude of said calibration signal, saidcurrent sources having a common control input for receiving saidcalibration signal, and each of said sources delivering its outputcurrent in accordance with the magnitude of said calibration signal. 6.The apparatus as claimed in claim 5, said contour plotting meansincluding a control for varying said contour voltage and selectivelysetting the latter at either a predetermined maximum amplitude or alesser amplitude which is a known percentage of the maximum amplitude.7. The apparatus as claimed in claim 6, said indicating means beingoperable to compare the amplitudes of the selected contour voltage andsaid sensed voltage and indicate when said amplitudes are equal.
 8. Theapparatus as claimed in claim 6, said calibration means being coupledwith said contour plotting means for increasing or decreasing thepredetermined maximum amplitude of said contour voltage as said selectedoutput current levels are decreased or increased respectively.
 9. Theapparatus as claimed in claim 3, the contact head of each of said pinshaving an annular contact surface in surrounding relationship to thepiercing axis of the pin.
 10. Apparatus for plotting an electricalanalog of a physical situation, said apparatus comprising: a resistivemedium adapted for modeling in accordance with the characteristics ofsaid situation, said medium comprising a sheet member, there being abacking member receiving said sheet member thereon and supporting thelatter; a hold-down frame shiftable to a position clamping the perimeterof the sheet member between the frame and the backing member; aplurality of input signal sources, each of which is provided with meansfor selectively setting its output at a predetermined level relative tothe other sources; a common return circuit coupled with the outputs ofsaid input signal sources and including a contact element carried bysaid frame and engaging the perimeter of the sheet member incircumscribing relationship thereto when the frame is in said position;a plurality of connectors for establishing electrical contact withselected contact points on said medium; means for coupling saidconnectors with the outputs of corresponding input signal sources forflow of currents along the medium from the connectors in contacttherewith, whereby voltages are developed in the medium in accordancewith the magnitudes and interaction of said currents; contour plottingmeans for providing a selectively variable contour voltage, and having avoltage sensing probe for contacting said medium and means forindicating the amplitude relationship between the sensed voltage and aselected contour voltage; and calibration means coupled with said inputsignal sources for varying the selected output levels thereof whilemaintaining the ratios of said selected levels constant, whereby theselected levels may be adjusted to normalize the selected contourvoltages by establishing the relative contour value of the point on themedium of highest potential.